This invention relates to improved processes and products for effecting laser-induced thermal transfer imaging. More specifically, the invention relates to a pigment image receiving layer having surface irregularities.
Laser-induced thermal transfer processes are well-known in applications such as color proofing and lithography. Such laser-induced processes include, for example, dye sublimation, dye transfer, melt transfer, and ablative material transfer. These processes have been described in, for example, Baldock, U.K. Patent No. 2,083,726; DeBoer, U.S. Pat. No. 4,942,141; Kellogg, U.S. Pat. No. 5,019,549; Evans, U.S. Pat. No. 4,948,776; Foley et al., U.S. Pat. No. 5,156,938; Ellis et al., U.S. Pat. No. 5,171,650; and Koshizuka et al., U.S. Pat. No. 4,643,917.
Laser-induced processes use a laserable assemblage comprising (a) a thermally imageable element that contains a thermally imageable layer, the exposed areas of which are transferred, and (b) a receiver element having an image receiving layer that is in contact with the thermally imageable layer. The laserable assemblage is imagewise exposed by a laser, usually an infrared laser, resulting in transfer of exposed areas of the thermally imageable layer from the thermally imageable element to the receiver element. The (imagewise) exposure takes place only in a small, selected region of the laserable assemblage at one time, so that transfer of material from the thermally imageable element to the receiver element can be built up one pixel at a time. Computer control produces transfer with high resolution and at high speed.
U.S. Pat. No. 5,902,770 discloses a thermal transfer image-receiving sheet having a dye-receptive layer having a surface roughness of center line average height Ra=1.0-4.0 microns and maximum height Rmax=15.0-37.0 microns. The thermal transfer sheets described in the ""770 patent are said to be useful in sublimation dye transfer using a sublimable dye and in a hot melt thermal transfer using a hot-melt ink layer comprising a hot-melt binder bearing a pigment.
U.S. Pat. No. 5,256,621 discloses a thermal transfer image-receiving sheet in which the surface of the dye image-receiving resinous layer has a surface roughness wave form with a maximum wave height (Rmax) of 1.0 or less at a wave length of 0.1 to 2 mm.
Micro-dropouts have been found to be a problem in thermal imaging processes utilizing fairly smooth image receiving sheets, wherein the surface roughness (Ra) is less than about 1xcexc, and wherein the thermal image transferred is a pigment image and not a dye-based image. A micro-dropout is an area that does not completely receive color from the pigment-containing thermally imageable element in the imaging process. The quality of the 4-color halftone image is superior from a visual standpoint when few to no micro-dropouts are present.
A need exists for roughened receivers that, when used in thermal imaging processes utilizing pigment-containing thermally imageable elements, give images wherein the micro-dropout problem is substantially eliminated.
The invention provides a thermal imaging process which substantially eliminates micro-dropouts.
In a first aspect of this invention a receiver element is provided for use in a thermal imaging process, wherein a surface of a pigment-image receiving layer of the receiver element has a roughness and the surface is brought into contact with a thermally imageable element, wherein the improvement comprises:
the pigment-image receiving layer provided on the receiver element having an average roughness (Ra) of less than about 1xcexc and surface irregularities having a plurality of peaks, at least about 40 of the peaks having a height of at least about 200 nm and a diameter of about 100 pixels over a surface area of about 458xcexc by about 602xcexc. By pigment-image receiving layer it is meant that the layer is capable of receiving a pigment image.
In the first aspect, the surface of the pigment-image receiving layer further comprises a gloss reading of about 5 to about 35 gloss units, more typically about to about 30 gloss units at an 85 degree angle.
In a second aspect, the invention provides a method for making a color image comprising:
(1) imagewise exposing to laser radiation a laserable assemblage comprising:
(A) a thermally imageable element comprising a thermally imageable pigment-containing layer; and
(B) a receiver element having a micro-roughened surface in contact with the thermally imageable layer; the receiver element comprising: a receiver support; a pigment-image receiving layer provided on the surface of the receiver support, the surface of the pigment-image receiving layer having an average roughness (Ra) of less than about 1xcexc and surface irregularities having a plurality of peaks, at least about 40 of the peaks having a height of at least about 200 nm and a diameter of about 100 pixels over a surface area of about 458xcexc by about 602xcexc; and whereby the exposed areas of the thermally imageable layer are transferred to the receiver element to form a pigment image on the pigment-image receiving layer; and
(2) separating the thermally imageable element (A) from the receiver element (B), thereby revealing the pigment image on the pigment-image receiving layer of the receiver element. This revealed pigment image may then be transferred directly to a permanent substrate such as paper or to a permanent substrate through an intermediate transfer step using an image rigidification element.
The roughness of the pigment-image receiving layer may be, for example, achieved by applying a micro-roughened sheet to the surface of the pigment-image receiving layer, typically with the application of pressure and optionally heat. It is important that the micro-roughened sheet that is used has a uniform roughness across its surface. Typically, the micro-roughened sheet has an average roughness (Ra) of about 1xcexc and surface irregularities having a plurality of peaks, at least about 20 of the peaks having a height of at least about 200 nm and a diameter of about 100 pixels over a surface area of about 458xcexc by about 602xcexc.